The invention relates to an all-wheel steering system for motor vehicles, with a randomly controllable front-wheel steering and rear-wheel steering which is positively coupled mechanically thereto.
An all-wheel steering system of this generic type is disclosed in German Patent document DE-OS 3,630,181, in which a link part is connected fixedly in terms of rotation to a cardan shaft which serves as a drive connection between the front-wheel and rear-wheel steering. The cardan shaft is positively coupled to the front-wheel steering via a crank arm arranged on the cardan shaft and via a linkage part articulated thereon. Gear elements, by which the transmission ratio between the steering angle of the front wheels and the steering angle of the rear wheels could be varied, however, are not provided.
A similar all-wheel steering arrangement is shown in Japanese Preliminary Publication 63-247170, in which a toothed pinion is positively coupled to the front-wheel steering via a cardan shaft. The toothed pinion itself drives a toothed quadrant connected fixedly in terms of rotation to an adjusting lever, the free end of which engages by means of a guide element into a slot arranged on the link part. Once again, however, there are no gear elements by which the transmission ratio between the steering angles of the front wheels and rear wheels could be varied.
German Patent Document DE-OS3,446,881 shows an all-wheel steering arrangement in which the rear-wheel steering is coupled mechanically to the front-wheel steering, and the transmission ratio between the steering angle of the front wheels and the rear wheels can be varied. A pivotable shaft part is arranged on an input shaft, positively coupled to the front-wheel steering, by means of a hinge-like fork joint, the hinge axis of which is aligned transversely relative to the axis of the input shaft. The end of the shaft part facing away from the fork joint is mounted rotatably on a connecting member, which is itself articulated on the track rods of the steerable rear wheels. By means of adjusting members, the connecting member can be moved vertically relative to the vehicle, in such a way that the shaft part is brought either into a position coaxial with the input shaft or into a position angled relative to the input shaft. As long as the input shaft and shaft parts are aligned coaxially with one another, a rotation of the input shaft cannot cause any movement of the connecting member, since the shaft part is rotationally mounted on the connecting member. However, when the shaft part is angled relative to the input shaft by pivoting about the axis of articulation of the hinge-like fork joint, during the rotation of the input shaft the shaft part acts in a similar way to a crank arm connected to the input shaft, and accordingly displaces the connecting member either to the right side or to the left side of the vehicle, depending on the direction of rotation of the input shaft, with the result that the steerable rear wheels are deflected correspondingly. Thus, the steering angle of the rear wheels depends both on the angle between the input shaft and the shaft part, and on the height setting of the connecting member. The transmission ratio between the front-wheel steering and the rear-wheel steering can thus be changed continuously.
A coupler mechanism of the all-wheel steering arrangement disclosed in German Patent Document DE-OS 3,446,881 affords the possibility of deflecting the rear wheels either in the same direction as the front wheels, but with a steering angle reduced in relation to the steering angle of the front wheels, or in the opposite direction to the front wheels. For this purpose, the connecting member merely has to be moved upwards or downwards by means of the adjusting members relative to its neutral position, in which the input shaft and shaft part aligned coaxially with one another.
It is thus basically possible to control the transmission ratio of the coupler mechanism as a function of driving speed, specifically in such a way that, at a very low speed such as is typical for parking maneuvers, the front and rear wheels are deflected in opposite directions in order to increase the maneuverability of the vehicle. Within a narrow range of moderate to medium speeds, the transmission ratio of the coupler mechanism usually has the value zero. (In this case, in the coupler mechanism of German Patent Document DE-OS 3,446,881, the input shaft and shaft part are aligned coaxially with one another.) That is to say the rear wheels remain constantly in their straight ahead position, irrespective of the particular steering angle of the front wheels. At higher speeds, especially also at maximum speed, the rear wheels can then be deflected in the same direction as the front wheels, but with steering angles smaller in comparison with the front wheels, particularly in order to increase the controllability of the vehicle during a rapid toe change or on "snaking" roads. Because the rear wheels are deflected in the same direction as the front wheels, a certain slip angle of the rear wheels is obtained at the very start of a steering maneuver, without a rotational movement of the vehicle about its vertical axis being necessary. The rear wheels can thus absorb high lateral forces at the very start of the steering maneuver.
In the transitional ranges between a very low and medium speed on the one hand and a medium speed and higher speed on the other hand, the transmission ratio of the coupler mechanism can be varied continuously. Thus, if the speed range with a deflection of the rear wheels in the opposite direction to the front wheels is assigned negative values of the transmission ratio, whilst the transmission ratio for the deflection of the rear wheels in the same direction as the front wheels is defined as positive, the transmission ratio increases more or less continuously with increasing driving speed.
It is known, in principle, to use electronic circuits for speed-dependent control of the transmission ratio of a coupler mechanism between a front-wheel and a rear-wheel steering. These circuits constantly carry out fault monitoring. Upon detection of a fault, separate asjustment members automatically adjust the coupler mechanism so that the transmission ratio has its maximum value and the front and rear wheels are always deflected in the same direction as one another.
In this emergency mode, driving safety is afforded unrestrictedly because the all-wheel steering behaves in the same way as at maximum speed, and with a fault-free transmission control of the coupler mechanism. However, the maneuverability of the vehicle is reduced because the deflection of the rear wheels in the same direction as the front wheels takes place even at a slow driving speed. In order nevertheless to avoid any unacceptable losses of maneuverability, all-wheel steering arrangements are often designed in such a way that the deflection of the rear wheels cannot exceed a limit value of, for example, 4.degree., even when the front wheels are deflected as far as their construction allows (for example by approximately 45.degree.). If appropriate, it is even possible for the steering angle of the rear wheels to decrease after the limit angle has been reached, or even for a deflection of the rear wheels in the opposite direction to the front wheels to take place when the front wheels are deflected exceptionally far.
Coupler mechanisms, albeit highly complicated ones, have already been designed, which, when a maximum transmission rataio is set between the deflection of the front wheels and the deflection of the rear wheels in the same direction as this, prevent the limit angle of the rear-wheel steering angle from being exceeded even when the front-wheel steering is subjected to an extremely hard lock. The coupler mechanisms designed hitherto are complicated, especially when the desired limitation of the rear-wheel steering angle is to be obtained solely by means of the kinematics of the coupler mechanism. In this case there must be a path dependent transmission ratio between the adjusting paths of the mechanism input and output.
In order to simplify the coupler mechanisms, spring elements (preferably prestressed), have been provided between the output of the coupler mechanism and the rear-wheel steering, and stops have been used to limit the steering angle of the rear wheels. Within the range determined by the stops, the adjusting stroke of the output of the coupler mechanism can then be transmitted virtually unchanged to the rear-wheel steering via the spring elements. When the stops are reached, and the front-wheel steering angle continues to increase, the steering angle of the rear wheels remains unchanged; that is, further movement of the coupler mechanism in response to increasing front-wheel steering angle simply exerts increasing tension of the spring elements, or causes an increase in the stop forces. The disadvantage of this concept however, is that in the transmission path between the rear-wheel steering and coupler mechanism there is a normally undesirable elasticity which, under high forces acting on the rear wheels from outside, can prevent an exact match between the steering movements of the rear wheels and the output of the coupler mechanism. Moreover, the spring elements tend to increase the steering resistance as soon as the stops limiting the steering angle of the rear wheels take effect.
Desirably simple mechanical coupler mechanisms satisfying all requirements have not yet been developed for all-wheel steering mechanisms.
For example, while German Patent Document DE-OS 3,820,967 shows an all-wheel steering arrangement in which the coupler mechanism between the front-wheel steering and rear-wheel steering has a relatively simple constructional design, it does not afford any possibility of limiting the rear-wheel steering angle when the front-wheel steering is at a very hard lock.
In principle, instead of mechanical coupler mechanisms, hydraulic coupling arrangements, or even electronically controlled systems working without any mechanical or hydraulic coupling, can also be provided for the rear-wheel steering. However, systems of this type offer a degree of safety comparable to that of mechanical coupler mechanisms only when multiple redundant essential elements are provided.
The object of the present invention is, therefore, to provide an all-wheel steering system with a positive mechanical coupling between the front and rear-wheel steering, which is especially simple in constructional design, but nevertheless makes it possible to obtain the desirable limitation of the rear-wheel steering angle solely by means of its kinematics.
According to the invention, this object is achieved in an all-wheel steering system having a front and rear-wheel coupler mechanism which is connected to the front-wheel steering, and which has a four-bar lever mechanism with two levers pivotable about stationary axes, and a link part which is connected to the free ends of the levers via pivotable joints. During the pivoting of the levers within a predetermined pivoting range, the link part executes a rotational movement in which a connection part arranged thereon describes a C-shaped or U-shaped path and displaces a rod, of which one end is articulated to the connection part and the other end is connected in driving terms to the rear-wheel steering. The rod is thus guided on a path which, in a top view of the C-shaped or U-shaped path, extends transversely relative to the C-legs or U-legs. The front-wheel steering is connected in driving terms to an input lever pivotable about a third stationary axis, and a further lever articulated adjustably on the free end of the input lever is coupled in an articulated manner to one lever of the four-bar lever mechanism via a connecting rod. The input lever, the further lever and the connecting rod have equal effective lengths, and in the straight-ahead position of the steering, the articulated connections between the connecting rod and the one lever and between the input lever and the further lever are located one above the other, with mutually aligned axes of articulation.
By adjusting the angle between the input lever and the further lever, the transmission ratio between the pivoting strokes of the input lever and of the one lever of the four-bar lever mechanism can be varied continuously, and a reversal of the relative directions of movement is also possible. Accordingly, depending on the set transmission ratio, the end of the rod located on the output side of the four-bar lever mechanism (that is, the end articulated to the connection part) covers distances of differing length on the C-shaped or U-shaped path when the input lever is pivoted through a predetermined angle. If the angle between the input lever and the further lever has the value zero, that is to say when the input lever and the further lever are located one above the other, as seen in the direction of the third stationary axis, then pivoting of the input lever causes no movement of the rod located on the output side of the four-bar lever mechanism.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.